1. Field
Communication systems, such as the long term evolution (LTE) of the third generation partnership project (3GPP), may interoperate with other communication systems. Interoperating communication systems may benefit from forking interworking in cases where, for example, one network supports session initiation protocol forking and one network does not.
2. Description of the Related Art
Session initiation protocol (SIP) forking is used in many networks. However, some networks do not support forking. As an example, the internet protocol (IP) multimedia subsystem (IMS) as described in 3GPP technical specification (TS) 23.228 and 3GPP TS 24.229 supports forking. By contrast, SIP with encapsulated integrated services digital network (ISDN) user part (ISUP) (SIP-I) based circuit switched core network (CS CN) discussed in 3GPP TS 23.231 and 3GPP TS 29.231 does not support forking.
To permit calls between networks that use forking and networks that do not support forking, appropriate signaling interworking procedures for the session initiation protocol (SIP), internet engineering task force (IETF) request for comments (RFC) 3261, and the session description protocol (SDP), IETF RFC 4566 and IETF RFC 3264, are required.
3GPP TS 29.235 describes procedures for interworking between the IMS and a SIP-I based CS CN performed by a media gateway control function (MGCF) as interworking unit. Some forking interworking related procedures are included, but those do not fully address SDP handling.